1. Field of the Invention
The present invention relates to a high area ratio, variable entrance geometry diffuser apparatus for use in converting high velocity gas, exciting a rotary compressor, to relatively low velocity, thereby converting kinetic energy to pressure energy, and a method for controlling mass flow rate through the compressor.
2. Description of the Prior Art
It is well known in the art of rotary compressors that most applications call for a reduction in the relatively high velocities of the gases exiting from such compressor apparatus for subsequent utilization, such as in power producing gas turbine engines. To achieve the conversion of the kinetic energy of the high velocity gases to a pressure increase in the gas, diffusers are currently employed downstream of the compressors to achieve the conversion via a subsonic diffusion process. Vane-type diffusers, diffusing scrolls and pipe or channel-type diffusers are the two principle types of apparatus conventionally utilized with rotary compressors to achieve the desired kinetic energy conversion.
Pipe-type compressor diffusers have an advantage over vane-type diffusers in that they can provide a better structural member for the compressor and related components in certain applications, such as gas turbine engines. Furthermore, as a result of the discrete spacing of such pipe-type diffusers about the axis of a rotary compressor, such diffusers allow for inter-channel spacings where various conduits for gas and oil can be passed for use elsewhere in the system. None of the above-mentioned diffusers can diffuse efficiently to an area ratio above about 4:1-5:1.
In connection with recuperated gas turbine engines it is especially important to have a highly efficient diffuser in order to achieve maximum pressure recovery of the high velocity gases emmanating from the compressor. In centrifugal compressors with a high pressure ratio the kinetic energy at the exit of a typical 4:1 area ratio diffuser represents 2-3 percentage points in isentropic efficiency and a further diffusion is desirable.